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The Rosetta mission to chase and land on a comet

Dr Amanda Doyle, Feature writer
Nov 10, 2014, 16:20 UTC

Sen—A comet dashing through the inner Solar System can provide quite a spectacle in the night sky. The ball of ice and rock bursts into life as the heat of the Sun triggers the formation of tails of gas and dust. But a comet is much more than a celestial display, it is also a treasure chest full of secrets.

Comets are remnants from the formation of the Solar System, and are believed to be almost unchanged since then. This makes them a perfect test bed to study ancient material that will reveal clues on both comet and planet formation. Chasing a comet is no easy task, and all space missions up until now have only been able to fly past comets.

ESA's Rosetta mission marks two firsts for space exploration; it is the first spacecraft to orbit a comet, and it will deploy the probe Philae for the first ever comet landing.

A decade to catch a comet
Rosetta was originally destined to rendezvous with comet 46P/Wirtanen, but a delay to the launch meant that the target had to be changed. On 2 March 2004, Rosetta was launched via an Ariane 5 rocket to chase down comet 67P/Churyumov-Gerasimenko.

Comet 67P is a short period comet which currently orbits the Sun every 6.45 years. It is thought to have originated in the Kuiper Belt, but its orbit is now heavily influenced by Jupiter's gravity.

Rosetta swung by the Earth three times for gravity assists, and also took a trip past Mars. In addition, Rosetta encountered two asteroids along the way. In 2008, the spacecraft flew by 2867 Steins, and in 2010 it passed 21 Lutetia.

The spacecraft is equipped with two solar panels, each of length 14 metres. The large solar panels mean that Rosetta is the first craft to fly in the region of Jupiter without relying heavily on other power sources. However, to reduce the consumption of power and fuel, the craft was placed in hibernation in June 2011.

Successfully awoken on 20 January 2014, Rosetta gradually began to resolve comet 67P as the craft sped ever closer. The first images revealed that the nucleus of the comet consists of two lobes that are stuck together.

"It is certainly an exciting looking comet", said Matt Taylor, project scientist for the Rosetta mission. "The comet has features from previous comets we have observed, so it appears to have everything we want rolled into one."

Rosetta finally caught up with comet 67P on 6 August 2014, and since moving to an orbit just ten kilometres above the surface, it has been returning stunning images. The craft has also been using its array of instruments to study the structure and composition of the comet.

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Rosetta OSIRIS wide-angle camera image of Comet 67P/Churyumov-Gerasimenko on 10 September 2014, showing jets of cometary activity along almost the entire body of the comet. Image credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/ INTA/UPM/DASP/IDA

Rosetta is capable of ultraviolet spectroscopy, which will be used to study gases in the coma and tail and determine the rate at which the comet produces water. It will also use other instruments to analyse the dust emitted by the comet and determine the amount, size, shape, and velocity of the dust grains emanating from the nucleus.

Rosetta will measure the temperature of the comet beneath the surface, and even study the interior by firing radio waves to penetrate the nucleus itself. This is the comet nucleus sounding experiment (CONSERT), and Philae is also equipped with a CONSERT instrument.

Radio waves are also used to measure the shifts in the spacecraft's signal, and thus determine the mass and density of the comet.

An ambitious landing
After arrival, the race began to select a landing site for Philae. Ten potential sites were selected, and this was eventually narrowed down to one. This site was initially known as Site J, however after a competition to name the site, Agilkia was chosen. Agilkia is situated on the smaller lobe and has the advantage of having less boulders than the other sites, as well as gentle slopes.

The most suitable time for Rosetta to deploy Philae is mid-November 2014, when the comet is three astronomical units from the Sun. Philae's battery will last for 64 hours, but it can be recharged via solar cells. Therefore, the comet needs to be close enough to the Sun for the landing site to be bathed in sunlight.

However, if the comet is too near the Sun, the risks increase for the lander. As the comet heats up, ice trapped beneath the surface will turn directly to gas and shoot out through cracks in the surface. This outgassing also brings dust grains with it, which could damage Philae.

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Philae's landing site located on the ‘head’ of Comet 67P/Churyumov–Gerasimenko. The mosaic comprises two images taken by Rosetta’s OSIRIS narrow-angle camera on 14 September 2014 from a distance of about 30 km. The image scale is 0.5 m/pixel. The circle is centred on the landing site and is approximately 500 m in radius. Image credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Rosetta will manoeuvre in front of the comet and on 12 November 2014 it will drop the lander. Philae will literally fall towards the comet at a walking pace of one metre per second. This low speed is necessary because the surface gravity of the comet is around one hundred thousand times less than the Earth. If Philae moved too eagerly, it would bounce off the comet and be lost to space.

The descent will be entirely automated, as the travel time for signals to the Earth is too long for the engineers to take control of the lander. Philae's journey will take about seven hours, during which the probe will snap shots of the surface and take measurements of the gas and dust close to the comet.

Upon landing, Philae will use ice screws at the bottom of each of its three legs to secure itself to the comet. It will also shoot out harpoons to act as anchors, and a thruster on top of the lander will counteract the recoil from the harpoon.

The lander is equipped with pre-programmed instructions for analysis, but once these results are relayed back to scientists on Earth, modifications or reanalyses of certain interesting things can be made.

Philae will provide a close-up view of the structure of the surface, and more importantly it will measure the composition directly. Philae can drill down to 23 centimetres, providing crucial data on the subsurface of the comet.

Philae could stay functioning up until March 2015, at which point the temperature on the comet's surface will become too hot for the probe to handle.

Escorting comet 67P
Rosetta will escort comet 67P to perihelion, which is the point in the comet's orbit that is closest to the Sun. This presents an exciting opportunity to study the comet as the activity increases, although the craft will have to move further away from comet 67P as it begins to spit out more material.

After perihelion in August 2015, Rosetta will continue to travel with comet 67P until the end of the nominal mission in December 2015.

"If we have enough fuel and the spacecraft is healthy and we get more funding to operate the spacecraft, we can in principle go beyond December 2015," Taylor told Sen. "Ultimately we will get to situation in 2016/17 when the spacecraft is too far from the sun, and would be in the same situation as it was in 2011 when it was put into hibernation."